1. Field of Invention
The field of the invention pertains to molding of thermoplastic materials, in particular, to the provision of improved part quality and replication for molded articles. The method and disclosed in this patent is applicable to common molding processes, i.e., injection molding, compression molding, blow molding, and thermoforming, wherein either a molten or malleable plastic material is placed inside the mold cavity or between two mold halves, shaped under pressure and then solidifies through cooling.
2. Description of Prior Art
Thermoplastic molding processes involve large and permanent deformation of the material at elevated temperature. It is common for conventional molding processes that the mold temperature stays at a preset constant temperature which is below the softening temperature of the material. Auxiliary heating and chilling equipment is usually employed to maintain this constant mold temperature. The so-configured manufacturing processes are robust and economically attractive due to very short cycle time, typically from several seconds to a minute, in producing common thermoplastic articles. However, these conventional molding processes also suffer from problems caused by great temperature difference between the material and the mold. Particularly in injection molding, the frozen layer resulting from such temperature difference is one of the primary causes for orientations, residual stresses, difficulty in molding thin sections, poor surface quality, and weak weld lines. In order to alleviate the premature freezing problem, a mold temperature close to or above the material softening temperature is need, thus resulting in intolerably long cycle time. For example, the cycle time for molding an aircraft shielding window using polycarbonate can be as long as several hours. The ideal molding condition would thus be a hot mold during the molding stage and a cold mold during the cooling stage. This ideal molding condition essentially uncouples the cooling process from the molding process and eliminates the processing contradiction.
To approach the hypothetical optimum molding condition, molding systems with very rapid heating and cooling capability on the time scale of the conventional cycle time are required. Since repeatedly heating and cooling a relatively massive mold requires considerable time and energy, means of heating only the mold surface are desired. This approach has been exemplified in the prior art as disclosed in U.S. patents.
U.S. Pat. No. 2,979,773 discloses a method and apparatus for heating the cavity surface wherein a semiconductive film is coated on the mold surface and an aluminum electrode plated into the semiconductive film.
U.S. Pat. No. 2,984,887 discloses a molding apparatus with an electrically heated copper or silver coating on the mold surface.
U.S. Pat. Nos. 3,671,168 and 3,763,293 disclose an approach to convectively heat the mold cavity using hot fluids.
U.S. Pat. No. 4,060,364 discloses an apparatus to produce molded plastic articles with a turntable carrying a plurality of molds through different stations. High-frequency dielectric heating is used to rapidly heat the plastic material.
U.S. Pat. No. 4,201,742 discloses a method for heating the mold cavity surface by means of condensing steam and draining the condensate from the cavity just prior to the injection stage.
U.S. Pat. No. 4,340,551 discloses a method to selectively heat only the superficial layer of the inner wall surface of the mold by high-frequency induction heating, to a temperature above the heat distortion temperature of the thermoplastics resin prior to injection molding.
U.S. Pat. No. 4,390,485 discloses a mold design that facilitates rapid heating and cooling by employing one or more thin metal surface sheets or bands of high electrical resistivity in the mold cavity heated using electrical current.
U.S. Pat. No. 4,442,061 discloses a heat cycle injection compression molding method wherein a temperature control apparatus with water and hot steam is used to reheat the injected plastic inside the mold cavity.
U.S. Pat. No. 4,548,773 discloses a mold design which features a cavity surface heated with electrical resistance heaters and controllably cooled by means of variable conductance heat pipe.
U.S. Pat. No. 4,800,049 describes a method and apparatus for manufacturing molded objects out of plastic material from a liquid plastisol which is injected into a mold subjected to high frequency dielectric heating.
U.S. Pat. Nos. 5,041,247, 5,176,839, 5,489,410, 5,535,980, and 5,728,474 that all assigned to General Electric Company disclose a multilayer mold that is passively heated by the incoming molten plastics.
U.S. Pat. No. 5,232,653 discloses a low thermal inertia mold with an embedded electrical resistivity heating unit and an embedded cooling system just beneath the mold surface.
U.S. Pat. No. 5,762,972 discloses an apparatus performing induction heating or dielectric heating of a mold for an injection molding system up to a desired temperature within a short time by using high frequencies or microwaves.
The use of systems like those disclosed in the prior art would presumably provide heat to a mold directly at the mold surface. One drawback of these methods, however, is their relatively low heating and cooling performance, i.e. low heated temperature, or slow thermal response. For example, one of the most popular methods, the passive heating method, which has been continuously improved over the past fifteen years and protected by many patents typically raises a temperature jump lower than 50° C. Active heating methods using direct current, alternating current, high frequency induced current, or microwave signals, such as those disclosed in U.S. Pat. Nos. 2,979,773, 2,984,887, 4,340,551, 4,390,485, and 5,762,972, theoretically could rapidly raise the mold surface temperature to over the polymer melting temperature. However, the reported heating rates are slow. For example only about 80° C./min was reported for an induction heated mold in U.S. Pat. No. 4,340,551. The reason for such low heating performance could be partially attributed to their relatively low thermal and mechanical durability and low heating efficiency.
The present invention discloses a new method for rapid mold heating based on surface heating generated by surface current due to proximity effect and/or skin effect on the mold insert powered by radio-frequency or high-frequency electrical current. When high frequency electrical current passing through a bulk or thick pair of electrical conductors which face with each other with a small gap in-between, the current will flow on the inner sides of the facing pair. This effect is referred to as the proximity effect in the application. When high frequency electrical current passing through a bulk or thick electrical conductor, the current will flow on the surface of the conduct. This effect is referred to as the skin effect in the application. The principle of the skin effect and the proximity effect is illustrated in FIG. 1.a and FIG. 1.b, respectively, and described and explained in detail in the DESCRIPTION OF THE INVENTION section.